It depends on the jurisdiction. Some IP offices notify existing trademark holders when a confusingly similar application is filed; others do not.
The EUIPO notifies owners of earlier marks if the examiner identifies a potential conflict during examination. Some national offices do the same. The USPTO does not proactively notify existing holders; the responsibility for monitoring rests entirely with the trademark owner.
Even in jurisdictions where notification is standard practice, it should not be relied upon as the only monitoring method. Examiner notifications are not guaranteed to capture every similar mark, and the responsibility for filing an opposition within the window remains with the owner.